As explained more fully in copending Application Ser. No. 411,054 entitled "Improvements in or relating to the sulfonation of aromatic compounds" of even date herewith, whose disclosure is incorporated herein by cross-reference, aromatic sulfonic acids, either as such or in the form of their salts with organic or inorganic bases, are widely used. Such aromatic sulfonic acids are usually prepared by reacting the corresponding unsulfonated aromatic compound with a sulfonating agent. In the past concentrated sulfuric acid has been the principle sulfonating agent employed, but oleum (which is a solution of sulfur trioxide in concentrated sulfuric acid) has also been used and in certain instances so too has sulfur trioxide itself.
However, the use of sulfuric acid always results in the production of water, which progressively dilutes the sulfuric acid employed, until its concentration becomes too low for it to effect sulfonation. Thus it will generally be necessary to employ a large excess of sulfuric acid over that theoretically needed, in order to effect a high conversion of the aromatic compound present into the corresponding sulfonic acid; but in that case, the end product is a mixture comprising the desired aromatic sulfonic acid, any unreacted aromatic compound, by-products of the reaction and excess sulfuric acid. The separation of these can be achieved only at some cost; and the substantial quantities of inorganic by-products (mainly sulfuric acid) can be disposed of only with difficulty, particularly now as effluent requirements become more stringent.
The use of oleum somewhat reduces these problems, since it is possible to use less oleum than sulfuric acid, but it will still normally be necessary to use an excess over that theoretically needed to react with the aromatic compound, and thus the problems of separating the desired sulfonic acid from the excess sulfuric acid and disposing of the latter still remain.
These problems can be more or less eliminated by using sulfur trioxide as the sulfonating agent. It reacts instantaneously with aromatic compounds, and it is not necessary to employ a substantial excess in order to achieve complete sulfonation. However, sulfur trioxide is very highly reactive, and its reactions with aromatic compounds are extremely exothermic and difficult to control; undesirable side reactions, particularly sulfone formation, often occur to a troublesome extent.
In attempts to overcome these drawbacks, and to moderate and control its reactions with aromatic compounds, the sulfur trioxide has been diluted with inert diluents before use. Two such procedures have been evolved; but one of them is only practicable for the sulfonation of hydrocarbons of high molecular weight (and thus very low vapor pressure at the reaction temperature) while the other is expensive to implement.
The first of these two known procedures using diluted sulfur trioxide involves diluting it with thoroughly dried air, to a concentration which is generally between 1 - 7% v/v; but this is unsuitable for sulfonating the more volatile aromatic compounds, since the amounts of such compounds entrained in the effluent air are so great as to be unacceptable, both from the point of view of cost and also because of the resultant toxicity hazards and fire and explosion risks, when the effluent air is discharged into the atmosphere -- such effluent air cannot be recycled, due to its contamination with small droplets of organic matter and sulfuric acid. Moreover, the volume of air used is large, and the drying of large volumes of air is expensive.
The other of these two known procedures using diluted sulfur trioxide involves diluting it with a solvent inert to all the reactants, and in which the sulfur trioxide dissolves. This process is not restricted to the sulfonation of the less volatile aromatic compounds; but it is inevitably expensive, since the solvent must be recovered after the reaction is completed, and some losses are inevitable. Moreover in practice very few solvents are satisfactory, since a suitable solvent must be completely inert vis-a-vis the reactants, and not only must dissolve sulfur trioxide but also should preferably dissolve both the aromatic compound to be sulfonated and the aromatic sulfonic acid obtained from it. Liquid sulfur dioxide meets all these requirements and has been the solvent generally used commercially hitherto; but its use inevitably requires expensive pressure equipment, and refrigeration plant is necessary for its recovery. It will be apparent therefore, that the use of this process will normally only be commercially justified for the manufacture of sulfonates which sell at comparatively high prices.
A further process involving the direct reaction of sulfur trioxide with the aromatic compound has also been developed, and involves maintaining a reduced pressure or near-vacuum conditions in the reaction zone, thereby increasing the inter-molecular spacing of the sulfur trioxide and hence decreasing the rate of reaction. The sulfonation by this method has been described of alkyl benzenes having from 11 to 15 carbon atoms in the alkyl group, which compounds have a comparatively low vapor pressure at the sulfonation temperature. This procedure is however inapplicable to the sulfonation of aromatic compounds having a high vapor pressure at the sulfonation temperature, since when using such compounds the quantity of vapor sucked away to the vacuum pump (provided to reduce the pressure) would be unacceptably high. Moreover, this process requires continuous cycling of the reaction mixture through a heat-exchanger to keep its temperature from rising to a point where side-reactions would become unacceptable.
In the aforesaid co-pending application Ser. No. 411,054 of even date herewith, there is disclosed a process where sulfur trioxide is passed into the aromatic substance in the liquid state at its boiling point, normally under reduced pressure. The highly exothermic reaction between sulfur trioxide and the aromatic substance is controlled in the process, as the boiling of the aromatic substance effects thorough mixing, thereby minimizing the dangers of local overheating, and the heat of reaction is removed from the reaction chamber as the latent heat of volatilization of the aromatic substance. However, the process described in the aforesaid co-pending application Ser. No. 411,054 is only suitable for the sulfonation of aromatic substances which boil under a pressure of 0.1 mm Hg at 100.degree.C or below. The sulfonation must be carried out at a temperature of 100.degree.C or below, since above this temperature side-reactions and darkening of the product take place to a significant extent, and thus this process can be applied only to aromatic substances which can be made to boil at 100.degree.C or below.